SLUG TESTS IN A REMOTE HIGH-ALTITUDE SETTING, NIWOT RIDGE, COLORADO

Knowledge of the distribution of hydraulic conductivity is crucial to understanding groundwater flow in the subsurface. The slug test is an efficient field method for determining hydraulic conductivity in aquifer material in the vicinity of a groundwater well. In particular, because the slug test does not require electricity or large amounts of equipment, it can be utilized in remote locations that are difficult to access such as high alpine regions.

For this study, numerous slugs tests were conducted in fourteen screened piezometers at two sites located on Niwot Ridge, a high-altitude ridgeline just east of the Continental Divide in the Front Range, Colorado. The piezometers are screened over 1.5 m at depths ranging from 2.1 - 9.1 m below the ground surface in either fractured igneous/metamorphic bedrock or poorly sorted sand and gravel. The goal of conducting these slug tests was to determine hydraulic conductivity of shallow aquifers on Niwot Ridge.

At least three tests were performed in each piezometer. The size of slug and type of slug (e.g. solid PVC or liquid water) were varied. Water slugs were used when the static water level was too low to fully submerge a PVC slug. Some tests were performed in summer after snowmelt when water levels were near their annual peak, and some were performed in fall during the dry season when water levels were declining.

All slug test results were analyzed using the solutions of Hvorslev, Cooper, and Bouwer and Rice methods to determine hydraulic conductivity of near-well aquifer material. The hydraulic conductivity for piezometers screened in fractured bedrock ranged from 0.42 - 3.95 m/d. For piezometers screened in poorly sorted sand and gravel, hydraulic conductivity ranged from 0.15 - 54.73 m/d. The range in results is consistent with reported ranges of hydraulic conductivity for earth materials. Results from solid PVC slug tests were within an order of magnitude of results from water slug tests. These results indicate that the slug test, using either a solid PVC or water slug, is a suitable method for determining hydraulic conductivity in locations where access and resources are limited. The hydraulic conductivity of the study area is characterized by higher values in the sand and gravel unit, lower values in the fractured bedrock, and generally higher values in the piezometers furthest west.